Of the many available electronic devices, operational amplifiers (op-amps) are some of the most widely used. Op-amps are efficient and versatile devices that can be used in a variety of applications, such as signal conditioning, analog instrumentation, analog computation, etc.
An op-amp may employ one of several different circuit arrangements. In one example, a class A amplifier reproduces an entire input signal because an active element of the class A amplifier, such as a transistor, is constantly in the active mode. Class A amplifiers typically have high power consumption because the active element constantly conducts current.
In another example, a class B amplifier typically employs two complementary output transistors, with each output transistor being turned on for half of the time and turned off for the other half of the time. That is, one output transistor operates as a current source, and the other output transistor operates as a current sink. This configuration is sometimes referred to as a “push-pull” configuration since a first branch of the output stage “pushes” or sources currents to a load while a second branch of the output stage “pulls” or sinks current from the load. A class B amplifier has lower power consumption than a class A amplifier but may be susceptible to crossover distortion due to the turn-on of one output transistor not matching the turn-off of the other output transistor, and may be unstable with capacitive loads at low currents.
A class AB amplifier also employs two complementary output transistors (similar to a class B amplifier). Class AB amplifiers avoid the high power consumption of a class A amplifier by always having one output branch substantially turn off when the other output branch is turned on. Although the current in one leg of a class AB amplifier is substantially turned off there is a small amount of current flowing in that leg. The small residual current in the class AB amplifier avoids the crossover distortion produced by the turning on and off of the currents in class B amplifiers. Thus, class AB amplifiers are able to achieve a relatively high current output while maintaining a low quiescent current and providing stability with capacitive loads at any load current.